Nuclear targeting of the CaMKII anchoring protein αKAP is regulated by alternative splicing and protein kinases

αKAP is an anchoring protein for the Ca2+/calmodulin-dependent protein kinase II (CaMKII) and is encoded within the same gene as the CaMKIIα isoform. αKAP co-assembles with CaMKII and targets such heteromers to the membrane of the sarcoplasmic reticulum, where CaMKII can regulate Ca2+ homeostasis. CaMKII has also nuclear functions in skeletal muscle, however, the nuclear targeting mechanism has been elusive. We show here that developmentally regulated splicing of exon EαB generates a functional nuclear localization signal (NLS) in αKAPB, the dominant αKAP variant in mature muscle. The αKAPA variant lacks the NLS and dominates in developing muscle before and around birth. Both αKAP variants localize to membranes, but a small fraction of αKAPB is additionally found in the nucleus. Indeed, α-karyopherins that mediate nuclear import bound to αKAPB but not αKAPA in vitro. When the N-terminal membrane anchor of αKAP was deleted, localization of αKAPB but not αKAPA became predominantly nuclear. Co-expression of constitutively active CaMKI and IV, which do not bind to αKAP, interfered with nuclear localization of αKAPB. CaMKIIα was found essentially exclusively in the cytoplasm when expressed in cell lines but was targeted to the nucleus when co-expressed with the nuclear form of αKAPB. Thus, nuclear targeting of cytoplasmic CaMKII isoforms by αKAP may be regulated by developmentally controlled alternative splicing and by protein kinases.